Effects of biochar‑based composite amendments on nitrogen migration and enzymatic activities in high‑nitrogen soils

High nitrogen leaching from farmlands with excessive residual nitrogen in the Erhai Lake Basin, combined with a lack of effective in situ retention measures, threatens aquatic ecological security. Hence, efficient and regionally adaptable soil amendments are urgently needed. A pot experiment was conducted with cabbage grown in high-nitrogen soil from the western shore of Erhai Lake to evaluate biochar-based composite amendments. The composites consisted of biochar (corn straw, rice husk, or bamboo), bentonite, and humic acid at three ratios (1 : 1 : 1, 2 : 1 : 1, 3 : 1 : 1) and were applied at 2.5% and 5.0% (w/w). The optimal treatment was 5.0% corn straw biochar composite at a 3 : 1 : 1 ratio (coded CSB6). Compared to the unamended control, CSB6 significantly increased soil total N, NO3–-N, and NH4+-N by 79.19%, 167.44%, and 200.73%, and reduced leachate total N, NO3–-N, and NH4+-N by 48.8%, 44.9%, and 67.95%, respectively. Urease, leucine aminopeptidase, and peroxidase activities were significantly enhanced by 125.63%, 55.70%, and 262.24%. The enhanced N retention and reduced leaching were attributed to synergistic physical adsorption, cation immobilisation, and complexation-mediated slow release. This study elucidates the regulatory mechanisms and optimal formulation, providing theoretical and technical support for source control of agricultural non-point N pollution in high-nitrogen farmlands of plateau lake basins.